The present invention relates to the design of pneumatic, radial ply, runflat passenger tires.
Various methods have been devised for enabling the safe continued operation of unpressurized or underpressurized vehicle tires with the intent of minimizing further damage to the uninflated tire and without simultaneously compromising vehicle handling over a distance from the place where the tire has lost its pressure to a place desired by the driver, such as a service station where the tire can be changed. Loss of tire pressure can result from a variety of causes, including puncture by a foreign object such as a nail or other sharp object piercing the pneumatic tire installed on a vehicle.
Pneumatic tires designed for sustained operation under conditions of unpressurization or underpressurization are also called runflat tires, as they are capable of being driven in the uninflated, or what would generally be called xe2x80x9cflat,xe2x80x9d condition. The conventional pneumatic tire collapses upon itself when it is uninflated and is carrying the weight of a vehicle. The tire""s sidewalls buckle outward in the circumferential portion of the tire where the tread contacts the ground, making the tire xe2x80x9cflat.xe2x80x9d
The term xe2x80x9crunflatxe2x80x9d is generally used to describe a tire that is designed such that the tire structure alone, in particular the structure of the sidewalls, has sufficient rigidity and strength to support the vehicle load when the tire is operated without being inflated. The sidewalls and internal surfaces of the tire do not collapse or buckle onto themselves, and the tire does not otherwise contain or use other supporting structures or devices to prevent the tire from collapsing.
An early example of a runflat tire design is described in U.S. Pat. No. 4,111,249, entitled the xe2x80x9cBanded Tire,xe2x80x9d in which a hoop or annular band approximately as wide as the tread is circumferentially deployed beneath the tread. The hoop in combination with the rest of the tire structure could support the vehicle weight in the uninflated condition.
Another approach taken has been simply to strengthen the sidewalls by increasing their cross-sectional thickness. However, due to the large amounts of rubber required to stiffen the sidewall members, flexure heating becomes a major factor in tire failure during runflat operation. This is especially so when the tire is operated for high speeds in the uninflated condition. Pirelli discloses such a runflat tire design in European Patent Publication No. 0-475-258A1.
In general, runflat tires incorporate sidewalls that are thicker and/or stiffer so that the tire""s load can be carried by an uninflated tire with minimum adverse effects upon the tire itself and upon vehicle handling until such reasonable time as the tire can be repaired or replaced. The typical methods used in sidewall thickening and stiffening include the incorporation of circumferentially disposed wedge inserts in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire usually having the lowest resistance to deformation under vertical loading. In such runflat tire designs, each sidewall is thickened in such a way that its overall thickness (gauge) is increased in the region between the bead and the tread shoulder.
The reinforced sidewalls of such tires, when operated in the uninflated condition, experience a net compressive load in the region of the sidewall that is closest to the road-contacting portion of the tire. Also, the bending stresses on the sidewalls are such that the outer portions of the reinforced sidewalls experience tensile forces while the inner portions experience compression stresses during runflat operation.
A Goodyear patent U.S. Pat. No. 5,368,082, by Oare et al, (""082), which has a common assignee and is incorporated in its entirety by reference herein, discloses a low aspect ratio runflat pneumatic radial ply tire which employs special sidewall inserts to improve stiffness. Approximately six additional pounds of weight per tire was required to support an 800 pound load in this uninflated tire. This earlier invention, although superior to prior attempts at runflat tire design, still imposed a weight penalty that could be offset by the elimination of a spare tire and the tire jack. However, this weight penalty becomes even more problematic in the design of tires having higher aspect ratios.
U.S. Pat. Nos. 5,427,166 and 5,511,599 of Walter L. Willard, Jr., disclose the addition of a third ply and the addition of a third insert in the sidewall of a tire to further increase the runflat performance of the tire over that of the U.S. Pat. No. 5,368,082 Oare et al. These two latter patents discuss some of the load relations that occur in the uninflated condition of the tire and they demonstrate that the Oare et al. concept can be applied to additional numbers of plies and inserts.
In general, runflat tire design is predicated upon the installation of reinforcing inserts inside each sidewall flex area. The inserts in each sidewall, in combination with the plies, add rigidity to the sidewalls in the absence of air pressure during runflat operation. The ""082 Patent teaches a sidewall construction for runflat tires in which the tire is constructed with two plies, an inner liner and two reinforcing wedge inserts in each sidewall. The two inserts in each sidewall are disposed such that one insert is located between the two plies while the other insert is located between the inner liner and the first or innermost ply.
While the high resistance to compression deflection of the inserts provides the necessary resistance to the collapse of the uninflated loaded tire, the use of multiple plies and, in each sidewall, more than one reinforcing wedge insert, has drawbacks which include the above mentioned increase in tire weight and flexure-induced heat build up. Such designs also increase the tire""s complexity in ways that adversely affect manufacturing and quality control.
U.S. Pat. No. 3,464,477 of Henri Verdier, and assigned to Michelin Corporation, discloses a pneumatic tire particularly for off-highway (OTR) use wherein the inflated tire is to be protected against damage such as cuts and abrasion to the sidewalls when the tire is used on rocky or rough ground. Although this is not a runflat design, there are useful teachings presented since the inventor has determined that sidewall damage from such OTR operation can be ameliorated by reinforcing the tire sidewalls: xe2x80x9csurprisingly . . . the sidewalls should be reinforced inwardly of the carcass plies.xe2x80x9d The disclosed reinforcement of each sidewall is a single reinforcing wedge insert (xe2x80x9celastomeric reinforcementxe2x80x9d) which has a maximum thickness at the mid height of the sidewall of the tire between about 1% and 3% of the overall maximum width of the tire, the reinforcement tapering toward its edges, that is toward the tread and toward the corresponding head of the sidewall and extending about half of the height of the tire.
While a high resistance to compressive deflection of the inserts provides the necessary resistance to collapse of the uninflated loaded tire, the use of multiple plies and more than one reinforcing wedge insert in each sidewall has drawbacks which include the increase in tire weight and cyclical-flexure-induced heating.
In the interests of operating efficiency and performance when operating with the tire normally inflated, the modifications to the sidewall and other areas of the tire for providing runflat operation should not adversely interact with the normal inflated operation of the tire. However, as the structure of the sidewall is stiffened with inserts and additional plies to provide runflat operation, some of the desirable operational characteristics of the tire in its inflated state are compromised.
The weight of additional sidewall plies and inserts resists the ability of the automobile suspension to allow the wheel to move in response to variations in the road surface. The increase in stiffness of the sidewall reduces the ability of the tire to comply with variations in the road surface. Both the increase in weight and radial stiffness may degrade the ride comfort and quality increasing harshness and noise perceived by the driver. Increases in tire weight associated with additional plies and inserts will also tend to reduce the performance of the automobile in straight line acceleration and deceleration. The increase in tire weight also tends to increase fuel consumption.
Clearly, the goal in runflat tire design is to provide a low-cost, light-weight tire that gives good runflat vehicle handling as well as good service life during runflat operation without compromising the durability, efficiency or performance of the tire during normal fully inflated operation.
The sidewalls of a fully inflated, conventional tire are thin, light weight and flexible compared to the sidewall of a runflat tire. The flexibility of the sidewall allows the tread area to move radially inward to allow the tire to absorb bumps. As a portion of tread area moves radially inward, the adjacent sidewall radius of curvature is decreased as the sidewall bulges out. This bending of the sidewall results in a relatively small amount of bending stress because the conventional sidewall is thin. Under normal inflated operation, radial deflection is resisted by the compressive force of the air in the tire. The compressed air provides a resistance to radial deflection that is more than proportional to the deflection. For example a 10% increase in radial deflection will require more than a 10% increase in the load. In effect, the overall modulus of elasticity of a conventional tire with respect to radial deflection is not constant but increases as the deflection decreases the volume of air contained by the tire.
The reinforced sidewalls of runflat tires resist the radial deflection with a combination of compressive and bending stresses in the inflated as well as the uninflated condition. Runflat fires experience a net compressive load in the region of the sidewall closest to the road-contacting portion of the tread. Also, the bending stresses on the thickened sidewalls are such that the outer portions of the sidewalls experience tensile forces while the inner portions experience compression stresses. Thus in addition to providing the structural rigidity required for runflat operation, the reinforced sidewalls of most runflat designs also reduce the desirable radial flexibility of the tire in the inflated condition. Accordingly, the reinforced sidewalls of runflat tires tend to detract from riding comfort, due to the increased structural rigidity of the tire.
It is apparent that there is a need to develop a runflat tire structure that provides the necessary structural rigidity in the uninflated condition, while also providing sufficient radial flexibility to optimize ride and handling characteristics in the normal inflated condition.
The present invention relates to a pneumatic radial ply runflat tire comprising a tread cap, two inextensible annular beads, a carcass comprising a radial ply structure, a gas-impervious inner liner, a belt structure located between the tread cap and the radial ply structure, two sidewalls each of which is reinforced with one or more wedge inserts, a pair of transition regions wherein the tread cap smoothly transitions into each sidewall, a pair of bead filler apexes, and a pair of chafers that protect the underlying ply structure from abrasion against a wheel rim having a rim flange. In a first embodiment, the runflat tire of the present invention has a radial ply structure that comprises at least two plies, and has at least two pairs of sidewall wedge inserts. In a second embodiment, the runflat tire of the present invention has a radial ply structure that comprises a single ply, and has one pair of sidewall wedge inserts. The invention can be generalized such that its scope includes all runflat tires having sidewalls with increased gauges (wall thicknesses) due to stiffening inserts and/or other stiffening devices such as fabric strips.
According to the present invention, the embodiments of the pneumatic radial ply runflat tire are characterized by tire wall gauges that constantly increase in thickness from the transition region to a bead/flange area where the chafer extends axially outward over the rim flange when the tire is mounted on the wheel rim.
According to the present invention, the embodiments of the pneumatic radial ply runflat tire may be further characterized by: a tire wall gauge being a shoulder gauge, measured in the middle of the transition regions; a tire wall gauge being a bead/flange gauge, measured immediately radially outward of the portion of the chafer that contacts the wheel rim flange; and a bead/flange gauge ratio, being defined as the bead/flange gauge divided by the shoulder gauge; wherein the bead/flange gauge ratio is within the range of 1.5 to 1.8 and preferably is approximately equal to 1.7. The embodiments of the pneumatic radial ply runflat tire may be further characterized by: a tire wall gauge being a mid-sidewall gauge, measured at the middle of the section height of the sidewall; and a mid-sidewall gauge ratio, being defined as the mid-sidewall gauge divided by the shoulder gauge; wherein the mid-sidewall gauge ratio is within the range of 1.1 to 1.4 and preferably is approximately equal to 1.3.
According to the present invention, the embodiments of the pneumatic radial ply runflat tire may be further characterized by a tire wall gauge being a shoulder gauge, measured in the middle of the transition regions; a tire wall gauge being a mid-sidewall gauge, measured at the middle of the section height of the sidewall; and a mid-sidewall gauge ratio, being defined as the mid-sidewall gauge divided by the shoulder gauge; wherein the mid-sidewall gauge ratio is within the range of 1.1 to 1.4 and preferably is approximately equal to 1.3.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.